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Week 16
MDAdvice.com Home > Health Library > Your Pregnancy: Week by Week > Previous Week > Contents > Next Week

How Big Is Your Baby?

The crown-to-rump length of your baby by this week is 4.3 to 4.6 inches (108 to 116mm). Weight is about 2.8 ounces (80g).

How Big Are You?

As your baby grows, your uterus and placenta are also growing. Six weeks ago, your uterus weighed about 5 ounces (140g). Today, it weighs about 8.75 ounces (250g). The amount of amniotic fluid around the baby is also increasing. There is now about 7.5 ounces (250ml) of fluid. You can easily feel your uterus about 3 inches (7.6cm) below your bellybutton.

How Your Baby Is Growing and Developing

There is fine lanugo hair on your baby's head. The umbilical cord is attached to the abdomen; this attachment has moved lower on the body of the fetus than before.

Fingernails are well formed. The illustration on page 139 shows legs are longer than arms. Arms and legs are moving. You can see this movement during an ultrasound examination. You may also be able to feel your baby move at this point in your pregnancy.

Many women describe feelings of movement as a ôgas bubbleö or ôfluttering.ö Often, it's something you have noticed for a few days or more, but you didn't realize what you were feeling. Then it occurs to you: You're feeling the baby moving inside you!

If you haven't felt your baby move yet, don't worry. Fetal movement, also called quickening, is usually felt between 16 and 20 weeks of pregnancy. The time is different for every woman. It can also be different from one pregnancy to another. One baby may be more active than another and produce more movement. The size of the baby or the number of fetuses can also affect what you feel.

Changes in You

Alpha-fetoprotein Testing

As your baby grows inside you, it produces alpha-fetoprotein. This protein is found in increasing amounts in the amniotic fluid coming from the fetal urine. Some alpha-fetoprotein crosses fetal membranes and enters your circulation. The amount of this protein found in your blood is much lower than in the fetus or the amniotic fluid, but it does increase as your pregnancy progresses.

It is possible to measure the amount of alpha-fetoprotein in the amniotic fluid (by amniocentesis) and in maternal blood (by drawing your blood). The level of this protein can be meaningful during pregnancy. An alpha-fetoprotein (AFP) test is usually done between 16 and 18 weeks of gestation. The timing of the test is important and must be correlated to the gestational age of your pregnancy and to your weight.

By this week, fine lanugo hair covers the baby's body and head.

An elevated level of alpha-fetoprotein can indicate problems with the fetus, such as spina bifida (spinal-cord problem) or anencephaly (serious central-nervous-system defect). Some researchers have even found an association between a low level of alpha-fetoprotein and Down syndrome. In the past, testing for Down syndrome could only be done with amniocentesis.

If the level of alpha-fetoprotein is abnormal, it may be confirmed with another alpha-fetoprotein test or by amniocentesis. A careful ultrasound examination is also done to look for spina bifida or anencephaly, and to determine how far along in pregnancy you are.

This test is not done on all pregnant women, although it is required in some states in the United States. It is not used routinely in Canada. If the test isn't offered to you, ask about it. There is relatively little risk to you, and it tells your doctor how your fetus is growing and developing.

Rh-Sensitivity

In the lab tests you've already had, your blood type and Rh-factor were determined. You may know this information by now. Your blood type, such as O, A, B, AB, and the Rh-factor are important. In the past, Rh-negative women who carried an Rh-positive child faced complicated pregnancies, which could result in a very sick baby.

Your blood is separate from your baby's blood. If you are Rh-positive, you don't have to worry about any of this. If you are Rh-negative, you need to know about it.

If you are Rh-negative and your baby is Rh-positive or if you have had a blood transfusion or received blood products of some kind, there's a risk you could become Rh-sensitized or isoimmunized. Isoimmunized means you make antibodies that circulate inside your system which don't harm you but can attack the Rh-positive blood of your growing baby. (If your baby is Rh-negative, there is no problem.) Your antibodies can cross the placenta and attack your baby's blood. This can cause blood disease of the fetus or newborn. It can make your baby very anemic while still inside the uterus, and it can be very serious.

Fortunately, this reaction is preventable. The use of RhoGAM (Rh-immune globulin) has alleviated many problems. It is given at 28 weeks gestation to prevent sensitization before delivery. Very few women are seen today who are sensitized. If you are Rh-negative and have a pregnancy, a RhoGAM injection should be part of your pregnancy. RhoGAM is a product that is extracted from human blood. If you have religious, ethical or personal reasons for not using blood or blood products, consult your physician and/or minister.

RhoGAM is also given to you within 72 hours after delivery, if your baby is Rh-positive. If your baby is Rh-negative, you don't need RhoGAM after delivery and you didn't need the shot during pregnancy. But it's better not to take that risk and to have the RhoGAM injection during pregnancy.

If you have an ectopic pregnancy and are Rh-negative, you should receive RhoGAM. This applies to miscarriage and abortions as well. If amniocentesis is performed during pregnancy and you are Rh-negative, you should receive RhoGAM.

How Your Actions Affect Your Baby's Development

Amniocentesis

If it is necessary, the amniocentesis test is usually performed for prenatal evaluation around 16 to 18 weeks of pregnancy. By this point, your uterus is large enough and there is enough fluid surrounding the baby to make the test possible. Doing the procedure at this time allows the woman enough time to make a decision about terminating the pregnancy, if that is what she desires.

With amniocentesis, ultrasound is used to locate a pocket of fluid where the fetus and placenta are not in the way. The abdomen over the uterus is cleaned. Skin is numbed, and a needle is placed through the abdominal wall into the uterus. Fluid is withdrawn from the amniotic cavity (area around the baby) with a syringe. About 1 ounce (30ml) of amniotic fluid is needed to perform various tests.

Fetal cells that float in the amniotic fluid can be grown in cultures. They are the cells used to identify fetal abnormalities. We know of more than 400 abnormalities a child can be born withùamniocentesis identifies about 40 (10%) of them. Abnormalities that can be identified include:

ò chromosomal problems, particularly Down syndrome

ò fetal sex, if sex-specific problems such as hemophilia must be identified

ò skeletal diseases, such as osteogenesis imperfecta

ò fetal infections, such as herpes or rubella

ò central-nervous-system diseases, such as anencephaly

ò hematologic (blood) diseases, such as erythroblastosis fetalis

ò inborn errors of metabolism (chemical problems or

deficiencies of enzymes), such as cystinuria or

maple-syrup-urine disease

Risks from amniocentesis include injury to the fetus, placenta or umbilical cord, infection, miscarriage or premature labor.

The use of ultrasound to guide the needle helps avoid complications but doesn't eliminate all risk. There can be bleeding from the fetus to the mother. This can be important because fetal and maternal blood are separate and may be different types. As I've just discussed, this is important to an Rh-negative mother carrying an Rh-positive baby. This type of bleeding can cause isoimmunization. An Rh-negative woman should receive RhoGAM at the time of amniocentesis to prevent isoimmunization.

Fetal loss from amniocentesis complications is estimated to be less than 3%. The procedure should be done only by someone who has experience doing it.

Saunas and Hot Tubs

Some women are concerned about using saunas, hot tubs and spas during pregnancy. They want to know if it is OK to relax in this way.

Your baby relies on you to maintain the correct body temperature. If your body temperature is elevated a few degrees and stays there for an extended period, it may damage the baby if it occurs at critical times in its development. Don't take a chance with a hot tub or sauna until more medical research can tell us it won't hurt your baby.

Tanning Booths

Phoebe always had a tan. She told me she maintained her tan at a tanning booth by regular weekly visits. She wanted to know if she could continue these visits during her pregnancy. I advised her that we don't yet know the effects on the growing fetus of a pregnant woman lying in a tanning booth. It has not been studied by medical researchers. ôUntil medical studies indicate it's safe to get in a tanning booth,ö I told Phoebe, ôit's best to avoid using them while you're pregnant.ö

You Should also Know

Change Sleeping Positions Now

Some women have questions and concerns about their sleeping positions and sleep habits while they're pregnant. Some want to know if they can sleep on their stomachs. Others want to know if they should stop sleeping on their waterbed.

As you grow during pregnancy, you'll find it more difficult to find comfortable positions to sleep. Don't lie on your back when you sleep. As your uterus gets larger, lying on your back can place the uterus on top of important blood vessels (the aorta and the inferior vena cava) that run down the back of your abdomen. This can decrease circulation to your baby and parts of your body. Some pregnant women also find it harder to breathe when lying on their backs.

Lying on your stomach when you sleep puts extra pressure on your growing uterus. This is another reason to learn to sleep on your side. Some women have commented to me that their favorite thing after delivery was to be able to sleep on their stomachs again!

Try to find comfortable ways to sleep. As time passes and you grow larger, it will become more important to you.

Tip for Week 16
Start now to learn to sleep on your side. It will pay off later as you get bigger. Sometimes it helps to use a few extra pillows. Put one behind you so if you roll onto your back, you won't lie flat. Put another pillow between your legs, or rest your ôtopö leg on it. Some manufacturers make a ôpregnancy pillowö that supports your entire body easily and effectively.

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From Your Pregnancy Week by Week by Glade B. Curtis, M.D., OB/GYN. Copyright by Fisher Books. Electronic rights by Medical Data Exchange.

 

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